proportions of the mortar for the brick work, are two parts of sand to one of hydraulic lime. The inverted arch of brick, as well as the brick facing on the sides, is four inches thick, and the roofing arch of brick is eight inches thick. After the masonry is finished the excavation which was done to receive it, is filled up around it, and over the top of the roofing arch generally to the height of 3 to 4 feet, and in some instances of deep excavation, up to the natural sur- face. If the natural surface be not of sufficient height for the top of the earth covering, the earth is raised to the re- quisite height with proper width on the top and slopes on the sides for protection to the Aqueduct masonry. Plate II. is a section of the Aqueduct in open cutting in rock. After the rock has been excavated to the required depth and width, the bottom is levelled up with concrete to the pro- per height and form for the inverted arch of brick, which is laid in the manner before described for earth excavation. The side walls of stone and brick are bonded together by headers of brick entering the stone walls as shown in the drawing, and the walls of stone are built closely against the sides of the rock and forming a junction with it. On the exterior of the roofing arch a heavy spandrel of stone ma- sonry (of the same character as the stone walls beneath it) is built, filling the space between the arch and the rock. After the masonry is finished, the rock cut above it is filled o'11' -,i"f miii^ini' ;;,r,,jj«,p: ::'';'':"^- !-/-■':•! ' 4§S '■!-:' t^i§^EKSS^3§5~ teisssssttEgSKg .i&feifefeS^fesassfeg i t 3 4 5 6 1 1 1 7 l I Scale of Feet * B Tat&\ Gxmber. Scale of Feet F B. Tower Jzmher W j- >~J~~ ^\ 1 i- f c- 1 2 3 4 S. Scale of Feet Scale of Feet Cun&tr. FlB Tower 85 with earth to the same height above the roofing arch as mentioned for earth excavation. Plate III. is a section of the Aqueduct in tunnel cutting in rock. The width of the tunnel excavation in rock is the same as that of open excavation in rock ; and the manner of build- ing the masonry to form the channel-way is the same, with the exception that the rock roof of the tunnel serves as the roof of the channel-way, where it is sound, but in cases where the rock is soft and liable to fall, a brick arch is built over the channel-way, and the space between its extrados, or outer surface, and the rock roof is filled with earth closely rammed in. In some instances where the tunnel perforated rock which was at first quite hard, the roofing has by expo- sure to the air, become soft and insecure, so as to render it necessary to turn an arch for its support. This is attended with inconvenience and some difficulty after the channel- way has been completed and closed through the tunnel. Plate IV. is a section of the Aqueduct in tunnel cutting in earth. When the earth is dry and compact, the excavation for the bottom and sides is made of a proper form to receive the masonry, which is built closely against it : the top is ex- cavated sufficiently high to give room to turn the arch, and the space above is afterwards filled with earth closely ram- med in. Where the earth is wet and there is difficulty in making it stand, the excavation is made larger, and props of timber and plank are used to support the top and sides until the masonry be completed ; and the whole space exterior to the masonry is then compactly filled with earth. 22 86 Plate V. is a section of the Aqueduct showing the man- ner of constructing it across valleys, or where the natural surface of the ground falls below the plane of grade. In such cases the Aqueduct is supported upon a founda- tion wall of stone laid dry, and formed by using large stones laid in positions to give proper bond, and to allow small broken stone to be closely packed in, filling up all the interstices so as to form a compact and uniform mass. The wall is generally allowed to stand some months after it is completed, before the masonry of the Aqueduct is commenced upon it, lest by this weight being placed upon it before it has found its bearing, it should settle and cause cracks in the masonry. That such settlement should in some instances occur, even after the Aqueduct is completed, is not surprising, for